This invention relates generally to beam index cathode ray tube (CRT) systems and particularly to apparatus for selecting and applying data from a source of data to an index type CRT.
U.S. Pat. No. 4,468,690 generally describes a beam index color display system including a FIFO memory for receiving data from a character generator or the like and for supplying that data for display on a beam index CRT under control of an index signal. One of the benefits of that patented system is that the character generator or source of data is not "slaved" to the memory. The present invention is directed to an improved version of an index system similar to that described in the patent. Specifically, it has been found that a much more efficient and economical system results when a pair of memories is employed, with each memory being capable of storing a full display line of pixel data or information. The benefits are obtained because the CRT display scanning system has a relatively long retrace time interval during which the electron beam is repositioned for display of the next line of data.
It will be appreciated by those skilled in the art that while normal television type display systems sweep an electron beam across the target or face of the CRT in a horizontal direction, it is envisioned, and indeed in some cases preferred, that the sweep direction be vertical rather than horizontal. Such a change will not affect the invention and systems employing vertical deflection as the primary deflection mode are intended to be encompassed herein.
In a beam index CRT, a single electron beam is scanned across a regular pattern or screen of colored light emissive phosphor strips that are interleaved with stripes of inert material. The back, or gun side, of the screen is "aluminized," that is, coated with a very thin layer of aluminum and strips of emissive material are positioned at regular intervals in overlying relationship to certain ones of the inert stripes. The emissive material, which may comprise a conventional monochrome type P47 phosphor, emits ultraviolet light when impacted by electrons from the electron beam, which light is picked up by a photocell or the like that is generally positioned at the rear and outside of the CRT. The signal developed by the photocell, which may be a photo multiplier tube (PMT), is processed to develop an index signal which, due to its periodicity and the regularity of the color phosphor stripe pattern, enables the position of the electron beam to be monitored in a very precise manner. The developed index signal is used to control application of appropriate video data to the color CRT for display.
The electron beam traces a "line" during its deflection or scan across the face of the CRT. At the beginning of the scan, a start or run-in signal is generated and subsequent emissions from the index strips serve to produce an index signal that locates the beam very precisely with respect to the CRT screen color strips. At the end of each scanned line, a retrace circuit returns the electron beam across the CRT face so that a subsequent line may be produced by scanning the electron beam from a slightly orthogonally displaced position on the CRT. As the electron beam is swept across the face of the CRT, the generated index signal controls the flow of video data that modulates the intensity of the electron beam to produce the desired color video display. Since no shadow mask is involved in a beam index CRT, the index signal is responsible for controlling application of the appropriate color video data for modulating the electron beam when it is positioned to impact a corresponding color phosphor stripe.
Conventionally, a computer, character generator or other source of video information supplies the color video data in a sequential manner. The data is stored as pixels at pixel address locations in a FIFO memory in the display system, and the information is "clocked out" of the memory by the index signal to modulate the electron beam of the CRT. The system does not provide a great deal of processing time and these restraints have an adverse cost impact. With the invention, a pair of addressable memories is utilized, each memory storing a complete scan line of data, and switching means are provided for alternately supplying data from one memory for controlling modulation of the electron beam in the CRT and for loading data into the other memory from a video data source.